We have described a phased core particle arising from in vitro association of a cloned segment of sea urchin DNA and the four smaller histones. We have now constructed a number of DNA fragments based on this sequence to allow study of (i) high resolution core particle structure, (ii) mechanisms involved in phasing of histone binding to DNA, and (iii) higher order structure of chromatin. Other investigations of nucleosome phasing have used a yeast plasmid containing a selectable gene product, a replication origin and, in some cases, insertions of non-yeast DNA of varying lengths. The structure of active genes has been investigated both in yeast plasmids and developing sea urchin embryos. In both cases, we find uniquely reversible features of chromatin organization which correlate with expression or repression of gene activity. Other systems are under investigation in looking at chromatin organization of transcriptionally regulated loci, including another sea urchin histone gene family, a lineage specific gene set in sea urchins, and the thionein gene of yeast.